Sternocleidomastoid Region: The “No Man’s Land” in Neck

Sternocleidomastoid muscle is an important structure of the neck. This rhomboid shaped muscle lies obliquely in the neck, extending from manubrium sterni, and medial third of the clavicle antero-inferiorly to the mastoid process and occipital bone postero-superiorly. The muscle superficially crosses various vital structures in the neck such as trachea, oesophagus, common carotid artery, internal jugular vein, vagus nerve, symphathetic chain and jugular group of lymph nodes. It is supplied by the occipital and superior thyroid arteries and receives its nerve supply from 11th cranial (Spinal root of accessory) and 2nd & 3rd cervical (Proprioceptive and motor) nerves. Sternocleidomastoid muscle separates the two triangles in each half of the neck [1] (Figure 1). The Anterior Triangle is described as being bounded by mid line anteriorly, anterior border of sternocleidomastoid posteriorly and the lower border of mandible superiorly. The Posterior triangle is described as bounded by posterior border of sternocleidomastoid, anterior border of trapezius and middle third of clavicle inferiorly.


Introduction
Sternocleidomastoid muscle is an important structure of the neck. This rhomboid shaped muscle lies obliquely in the neck, extending from manubrium sterni, and medial third of the clavicle antero-inferiorly to the mastoid process and occipital bone postero-superiorly. The muscle superficially crosses various vital structures in the neck such as trachea, oesophagus, common carotid artery, internal jugular vein, vagus nerve, symphathetic chain and jugular group of lymph nodes. It is supplied by the occipital and superior thyroid arteries and receives its nerve supply from 11 th cranial (Spinal root of accessory) and 2 nd & 3 rd cervical (Proprioceptive and motor) nerves. Sternocleidomastoid muscle separates the two triangles in each half of the neck [1] (Figure 1). The Anterior Triangle is described as being bounded by mid line anteriorly, anterior border of sternocleidomastoid posteriorly and the lower border of mandible superiorly. The Posterior triangle is described as bounded by posterior border of sternocleidomastoid, anterior border of trapezius and middle third of clavicle inferiorly.     In the upper part a.
In the middle part The point at the junction of the upper and middle third of the posterior border of sternocleidomastoid where four cutaneous nerves and spinal accessory nerve emerge is termed "nerve point of the neck". In 'cervical plexus nerve block' the anesthetic agent is injected at this site [2,3].
Trauma surgeons divide neck region into zone 1, zone 2 and zone 3. Anatomic contents of different zones of neck [4] are shown in Figure 3

Discussion
Sternocleidomastoid is a long muscle which acts as an important landmark in the neck as it divides the neck into anterior and posterior triangles. The Anterior Triangle is described as being bounded by mid line anteriorly, anterior border of sternocleidomastoid posteriorly and the lower border of mandible superiorly. The Posterior triangle is described as bounded by posterior border of sternocleidomastoid, anterior border of trapezius and middle third of clavicle inferiorly. Inferiorly, the muscle has two heads, tendinous sternal head attached to manubrium sterni and muscular clavicular head attached to medial third of the clavicle (clavicular head). Superiorly, muscle is attached to the mastoid process of the temporal bone and superior nuchal line of the occipital bone. The sternal head ascends across the medial part of the sternoclavicular joint, and widens as it overlaps the clavicular head a short distance above the clavicle, fusing with it about half way up the neck. In the postero-superior part its anterior border becomes thick before it inserts into the anterior surface of the mastoid process while the posterior border becomes thin and aponeurotic before attaching to the lateral surface of the mastoid process and lateral half of the superior nuchal line [5].
The width of the muscle could be up to 4-5 cm in muscular men. It is supplied by three sternocleidomastoid arteries. Out of which upper two arise from the occipital artery and the lower one arises from the superior thyroid artery. The carotid sheath with its contents is overlapped by sternocleidomastoid muscle in most of its extent, however in literature it is described as a content of anterior triangle. This indistinct demarcation thus causes dilemma in understanding the applied anatomy of this region. The clinician, the radiologist and surgeons have to be cognizant of the vital structures located deep to sternocleidomastoid muscle. Surgical excision of any swelling along its length such as branchial cyst or enlarged lymph nodes needs precise understanding of this region as the structures injured and surgical approach varies in different zones. Trauma surgeons divide the neck into three zones (zone 1, zone 2 and zone 3) extending from skull base to the sternal notch based on the urgency of exploration. The triangle based description of neck anatomy is not inclusive of this important region of the neck, "The sternocleidomastoid region". This review thus highlights the deficiency in teaching of head and neck anatomy.

a.
We should teach the anatomy of neck as consisting of two triangles separated by a rhomboid. The rhomboid is the "sternocleidomastoid region". b.
The anatomy of sternocleidomastoid region is of great significance to the clinician because correct diagnosis of neck swellings requires thorough knowledge of structures located in this region.

c.
A radiologist evaluating the neck during ultrasound, CT scan or MRI again requires understanding of the above structures. d.
Surgeons operating upon lateral neck swelling have to be familiar with the sternocleidomastoid region. e.
Operative procedure of neck dissection for malignancies requires retraction of the sternocleidomastoid in order to dissect and remove all the lymph nodes of the jugular chain.

f.
Sternocleidomastoid region, an area up to 5cm in width deserves a special mention in the anatomy of the neck.